The present invention relates to signal processing and, more particularly, to techniques for estimating certain parameters by means of processing a received signal containing possible contributions from known signals originating from multiple sources.
In general, before a known signal is received by a receiver, the known signal is modified by effects due to various receiver parameters such as the receiver""s spatio-temporal location, the receiver""s velocity and the offset and drift rate of the receiver""s clock. The receiver""s location affects the signal delay, that is the unknown amount of time by which the signal is delayed in the course of traveling from the source to the receiver. In addition, the carrier frequency of the known signal is shifted by an unknown amount due to the receiver""s clock drift and the Doppler shift caused by the relative motion of the receiver with respect to the source. Clock drift is herein referred to as xe2x80x9cclock Dopplerxe2x80x9d. The overall frequency shift, herein referred to as xe2x80x9cDoppler shiftxe2x80x9d, is the sum of clock Doppler and motion Doppler. The motion Doppler is further decomposed into source motion Doppler and receiver motion Doppler. In many applications, the source motion Doppler is known and the unknown Doppler shift depends only on unknown receiver parameters, such as clock Doppler and receiver motion Doppler.
The signal that is received at the receiver, which typically includes noise, is generally referred to as the xe2x80x9creceived signalxe2x80x9d, or also xe2x80x9cdataxe2x80x9d. For many applications, when a receiver samples the received signal for a long duration of time, it is important to know the carrier frequency of the received signal to a high degree of accuracy in order to process the received signal using coherent correlation methods. In particular, for signals sampled over a long duration, an inaccurate estimation of the carrier frequency will often result in out-of-phase correlations. Estimation of the carrier frequency is equivalent to estimation of the Doppler shift. The carrier frequency and delay of a known signal are herein referred to also as xe2x80x9csignal parametersxe2x80x9d.
Typically, in the case of multiple known signals being transmitted to a receiver by multiple sources, as in the case of Global Positioning System, the Doppler shifts and delay values of the received signals are independently calculated for each satellite vehicle that is potentially overhead the receiver.
A significant disadvantage to the above approach is that the approach does not leverage the calculations for the signal and receiver parameters performed with respect to one source against the calculations with respect to another source, thus resulting in a high computational expense. Further, the approach does not take advantage of the fact that it may be possible to obtain a coarse estimate of certain signal and receiver parameters with small computational expense by processing a small segment of the received signal. A coarse estimate obtained from a small data segment can be used to significantly reduce the computational expense in processing a larger data segment and/or the entire segment of received data.
Based on the foregoing, there is a clear need for a technique to determine the receiver parameters and the carrier frequencies and delays of the known signals that leverages the information gathered from a subset of the sources and from shorter segments of the received signal to reduce the calculations needed to process the entire duration of the received signal and the entire set of sources.
Techniques are provided for determining the spatio-temporal and kinematic receiver parameters and, in addition, the Doppler shifts and the signal delays (the signal parameters) of multiple known signals transmitted by multiple sources. One aspect of the invention is to divide the sampled data received by the receiver into possibly overlapping segments of data, herein referred to as xe2x80x9cdata segmentsxe2x80x9d. In some embodiments, these segments form a nested sequence of segments of increasing length. Receiver and signal parameters are estimated by searching for the known signals in the various data segments. Each pair consisting of a source and a data segment, herein referred to as xe2x80x9csource-segmentxe2x80x9d pair, yields a piece of information obtained by means of searching for the known signal corresponding to that source in that data segment. In some embodiments, the search is carried out by correlating data segments with the known signals for a suitable set of signal parameter values. The information recovered from the source-segment pairs that have already been processed is used to reduce and refine the calculations for the other source-segment pairs.
A xe2x80x9cfusion enginexe2x80x9d is used to store and integrate the information on receiver parameters and signal parameters gathered at any intermediate stage from the source-segment pairs that have been processed. Such information is represented inside the fusion engine by means of an xe2x80x9cuncertainty region representationxe2x80x9d, which in some embodiments consists of a set of inequalities. At any stage, the engine uses the current uncertainty region representation to generate estimates and bounds of signal parameters (such as Doppler shifts and delay values) used to guide the search in the subsequent stages. Estimates and bounds of signal parameters and receiver parameters get refined as more stages are completed.